1. Field of the Invention
The present invention relates to a network system for communication of a signal, particularly a packet, among terminals connected thereto, a node device adapted for use in said network system and a communication method adapted for use in said network system. In particular, said network system employs a multi-channel transmission path.
2. Related Background Art
With the recent progress in the processing speed of terminals, there is being investigated a network system, utilizing a network consisting of multi-channel transmission paths, for realizing a faster process in the network connecting such terminals. As shown in FIG. 1, such network system is composed of node devices 120 for connecting plural terminals 126, 127, and a multi-channel transmission path 128 with plural channels, for connecting said node devices. In FIG. 1, the multi-channel transmission path consists, as an example, of a wavelength multiplex transmission path employing plural wavelengths.
In the network system shown in FIG. 1, a packet transmitted from the terminal 126 and entered into an input I/F unit 124 is switched in an exchange unit 122 to one of plural fixed wavelength transmission units 123 and is transmitted therein with a predetermined wavelength. It is then subjected to a relay process by any node device present in the path to a node device to which a destination terminal is connected. It is finally received by a fixed wavelength reception unit 121 of the destination node device, then is switched by an exchange unit so as to be released from an output I/F unit 125 to which a destination terminal is connected, thus released from the predetermined output I/F unit and received by the terminal. The exchange unit 122 of the node device sets the route for the packet to the desired terminal of the desired node device, by an exchange operation for sending the input packet to one of plural fixed wavelength transmission units and plural I/F units.
In such conventional configuration, the exchange unit is constructed in the following manner.
FIG. 2 shows an example of configuration of the exchange unit for use in such conventional example, representing a cross-bar exchange with N inputs and N outputs, wherein said number N of the inputs is equal to the sum of the number of the fixed wavelength reception units and the number of the input I/F units, and the number N of the outputs is equal to the sum of the number of the fixed wavelength transmission units and the number of the output I/F units.
In FIG. 2, 129 indicates a decoder for reading an address section of the packet and generating output designation data for indicating the destination of the packet to a control unit. 130 indicates a FIFO (first-in-first-out) register for temporarily storing the input packet and releasing said packet in the order of input to an output line, under the control by the control unit. 131-1 to 131-n indicate input lines serving to supply packet signals, released from the FIFO registers, to switch input terminals. 132 indicates a switch for switching whether or not to supply the packet signal, supplied from an input line, to an output line. For an input number N and an output number N, there are required N.times.N switches. 133 indicates a control unit for effecting read-out control of the FIFO registers and control of the switches, according to the output designation data from the decoder. 134-1 to 134-n indicate output lines for supplying the packet signals, released from the switches, to respective output units.
FIG. 3 shows the configuration of the packet exchanged in such packet exchange devices, and said packet is composed of an address section 135 indicating the destination terminal of the packet and a data section 136 indicating the data transmitted by said packet.
FIG. 4 shows an example of the decoder unit for decoding addresses of all (n) terminals connected to the network system.
Referring to FIG. 4, a latch 137 temporarily stores the destination address section of the input packet. Memories 138 of a number n respectively store, in advance by an unrepresented decoder management unit, addresses of all the terminals of a number n connected to the network system. Each of comparators 139 compares the destination address of the packet temporarily stored in the latch, with the address stored in each memory, and, in case of coincidence, sends a coincidence signal to a table address generator 140, which generates a table address for reading an output designation table. An output designation table 141 stores output designation data indicating the on-off operations of the switches. In response to an instruction from the table address generator, desired output designation data are read from said table and supplied to the control unit 133. The table address generated by the table address generator is to read data of the table corresponding to the number of the comparator which has generated the coincidence signal, so that the output designation data are read from the table corresponding to the destination address of the entered packet. Based on said output designation data, the switches 132 are operated and the packet entered from the input terminal is released from a predetermined output terminal.
As explained in the foregoing, the conventional configuration has been associated with a first drawback that a high cost is required for connection of the terminal to the network, as each node device has to be equipped with an expensive exchange unit with a large hardware magnitude.
Also the exchange unit requires N.times.N switches for N inputs and N outputs, and said number N of inputs is selected equal to the sum of the number of the fixed wavelength reception units and that of the input I/F units while said number N of outputs is selected equal to the sum of the number of the fixed wavelength transmission units and that of the output I/F units. Consequently there has existed a second drawback that the number of switches increases with a rate of square and the magnitude of hardware increases significantly with the increase in the number of the channels in the multi-channel transmission path or of the connected terminals.
Furthermore there has existed another drawback as explained in the following. In the decoder of the conventional configuration, as explained in the foregoing, the destination address of the packet is compared with the addresses of all the terminals connected to the network system, and the output designation data are read from the output designation table based on the coinciding terminal address. Consequently there are required sets of memory and comparator of a number at least equal to the number of the terminals connected to the network, and there are also required output designation tables of a same number. Also the table address generator requires a longer time for generating the table address, with the increase in the number of the coincidence signals to be entered. Consequently the decoder of the conventional configuration has been associated with a third drawback that, with the increase in the number of the terminals connected to the network system, the node device becomes expensive because of the increased hardware magnitude and the network system becomes unable to operate at a high speed as the high-speed address decoding becomes difficult.